Resistance-capacitance tuned oscillator



Aug. 10, 1948. w. GAssEL ETAL RESISTANCE-CAPACITANCE TUNED OSCILLATORFiled Dec. 30, 1944 Patented Aug. l0, 1948 RESISTANCE OAPACITANOE TUNEDOSCILLATOR William Gassel, Philadelphia, Pa., and Henry Gritz, Iuka,Miss., assignors to Philco Corporation, Philadelphia, Pa., a,corporation of Penn- Sylvania Application December 30, 1944, Serial No.570,678

2 Claims. .l Y

The invention relates to oscillation generators and more particularly toimprovements in resistance-capacitance tuned oscillators.

As is well known, the tuning range for any one frequency band of aconventional resistance-capacitance tuned oscillator has a maximum tominimum frequency ratio of approximately tento-one. In order to generateoscillations over a wide frequency range, such as obtains from apractical wide range audio oscillator of this type, it is, at present,common practice to provide the oscillator with a suicient number offrequency bands to cover the desired frequency span. To this end, thereis usually included in the frequency-determining network of theoscillator several sets of resistors, together with a switching meansfor connecting a particular set of resistors in the network circuit foreach frequency band.

It is evident that certain advantages procure from an oscillator whosefrequency may be varied in a continuous and smooth manner over a Widerange by one tuning control, rather than in a step-like fashionresulting from a plurality of frequency bands with the involvedswitching mechanism.

It is, therefore, the principal object of this invention to provide animproved resistance-capacitance tuned oscillator capable of generating awide range of frequencies on one frequency band.

Another object of the present invention is to provide a novel and simplemeans whereby the usual tuning control of a resistance-capacitance tunedoscillator may vary the oscillator frequency over a Wide range.

Another object of the present invention is to provide a novel wide rangeaudio oscillator.

A further object of t'his invention is to provide a novelfrequency-determining network for a wide range resistance-capacitancetuned oscillator.

Other objects and advantages of the present invention will becomeapparent during the course of the following description taken inconjunction with accompanying drawing in which Fig 1 shows schematicallyone embodiment of the present invention applied to one of the morecommon types of conventional resistance-capacitance tuned oscillators;and

Fig. 2 shows a modification.

In Fig. 1 of the drawing, that portion of the circuit including thetwo-stage resistance-coupled amplifier represented by the block I' andthe regenerative coupling network composed of resistors R1 and R2 andvariable condensers C1 and C2 comprises one form of -a conventionalresistance-capacitance tuned oscillator. Since the oscillator frequencydepends upon the resistance of resistors R1 and R2 and the setting ofvariable condensers C1 and C2 the above network is often referred to asthe frequency-determining network. In accordance with usual practice,the amplifier should preferably be provided with a suitable inversefeedback and amplitude control circuit to insure an oscillator outputhaving good wave form. In practical oscillators of this type, the tworesistors R1 and R2 are usually of the same value, and the two variablecondensers C1 and C2 are similar and are ganged as illustrated in thedrawing.

For each value of resistance for resistors R1 and R2, the frequency ofthe oscillator may be varied, by means of the tuning control attached tothe ganged variable condensers C1 and C2, over a band whose frequencyratio is about 10 to 1.

In accordance with the present invention, the tuning control of aresistance-capacitance tuned oscillator is caused to Vary the oscillatorfrequency over a considerably increased frequency range. This isaccomplished by means of the series-connected resistance-capacitanceelements RaCa and R404, which are respectively connected in shuntrelation with resistors R1 and R2. Each of the condensers C3 and C4 isof small capacity, and the resistance of resistors R3 and R4 is low incomparison to that of resistors R1 and R2. As the oscillator frequencyis increased, the consequent decrease in the reactance of condensers Czand C4 allows the resistors R3 and R4 to have more and more shuntingeffect upon the resistors R1 and R1 respectively, thereby lowering theeffective resistance across these latter two resistors. Variation ofboth the capacity and the effective resistance of thefrequency-determining network during oscillator tuning permits thetuning control to govern the oscillator frequency over a wide range offrequencies. By way of example, in an audio oscillator constructed inaccordance with the embodiment of the invention illustrated in thedrawing, a tuning range of 400 to 1 was attained. By selecting theproper element values for each resistance-capacitance mesh RaCa andResistors R1 and R2 8.2 megohms each CondensersCi and C2 525 auf, eachResistor1R3 180,000 ohms Condenser C3 75 auf.

Resistor R4 220,000 ohms Condenser C4 60 [.Lpnf.

It should be noted that this invention provides*V a means of Calibratingthe middle of the oscillator .scale without affecting the calibration ofthe high :or low ends of the scale. To this. end`,.a padder :condensermay be connected' across capacitor C. :as shown. It will be understood,of course, that :this element is optional. The calibration of theoscillator at the high end of the scale may, ofY

,course, be accomplished in the usual manner by :a padder condenser onvariable capacitor C1. Am- Aplitude control may also be obtained intheusual :manner by a padder condenser on variable capac- Although theinvention has been described with reference tothe embodiment shown inFig. 1 of the drawing, it is to be understood that the invention is notlimited to the particular values given by way' of example, nor to thespecific number and arrangement of the elements in theresistance-capa'citance networks, represented in the drawing by R303 andR4C4. For instance, the

values of resistance employed for resistors R1 and R2 may be such thatthe resistance of Rr is considerably different from that of R2.

Also, improvement in` the wave shape of the output voltage and thefrequency response characteristic of the oscillator over its widefrequency range may be obtained by connecting a resistor 2 acrosscapacitor C3, or by shunting a capacitor 3kacr0ss resistor R1, or,preferably, by employing both as shown in Fig. 2. Resistor 2 should beof sufcient resistance to have minimum effect upon the lowest frequencydesired from the oscillator. Condenser 3 should be-of small -capacity inorder that the higher frequency range of the oscillator will not beaffected. The improved wave form is occasioned by the increase indegenerative feedback which may be applied to the amplifier l as aresult of the increase in regenerative feedback obtained through theseadditional elements.

With resistor 2 and capacitor 3 in the regenerative feedback circuitthere is also an increase in the amplitude of the output voltage in thelower middle portion of the frequency range thus compensating for a dropof voltage occurring at this part of the range when these elements arenot used. Aside from these additional elements, Fig. 2 is similar toFig. 1.

It will thus be apparent to those skilled in the art that the presentinvention is capable of various embodiments or modifications which maybe resorted to without departing from its scope.

We claim:

1. In an oscillator circuit, an amplifier, a regenerative couplingnetwork connected between the output and input of said amplier, saidnetworkincluding a resistor and a variable capacitor connected in seriesbetween the output and input of said amplier, .and also including asecond resistox. and a second variable capacitor connected in shuntrelation across the input of said amplifier, and series-connectedresistance and capacitance elements in shunt with each of saidresistors. and adapted to vary the effective resistance of' said networkin response to variation of the oscillator frequency, whereby toincrease the frequency range of the oscillator.

2. In an oscillator circuit, an amplifier, a regenerative -couplingnetwork connected between the output and input of said amplifier, saidnetwork comprising a rst branch including a resistor and a variablecapacitor connected in series between the output and input of saidamplifier and a second branch also including a resistor and a variablecapacitor connected in shunt relation across the input of saidamplifier,and seriesconnected resistance and capacitance elements'in shunt with`at least one of said resistors and adapted to vary the effectiveresistance of one of said branches in response tothe variation of theoscillator frequency wherebyA to increase the frequency range of theoscillator.

" WILLIAM GASSEL.

HENRY GRII'Z.

REFERENCES CITED The following references are of recordv in ther illeofthis patent:

UNITED STATES PATENTS Number Name Date 1,442,781 Nichols Jan. 16, 19232,024,489 Pal Dec. 17, 1935 2,268,872 Hewlett Jan. 6, 1942 2,356,248'Koren Aug. 22, 1944

